High strength galvanized steel sheets for use in automotive parts etc., are required to exhibit excellent formability as well as high strength due to their characteristic usage.
In recent years, there are increasing pressures for better automotive fuel efficiency in order to regulate the CO2 emission from the viewpoint of global environmental protection. Furthermore, in order to ensure safety of occupants at the time of crash, there are also increasing pressures for higher safety mainly concerning crash safety properties of automotive bodies. Due to these concerns, weight reduction of automotive bodies and strengthening of automotive bodies have been actively pursued.
In order to reduce weight and increase strength of automotive bodies at the same time, it is allegedly effective to increase strength of parts material and decrease weight by decreasing the sheet thickness by an amount that does not cause problems associated with rigidity. Recently, high strength steel sheets have been increasingly employed in automotive parts and steel sheets used as structural parts and reinforcing parts of automotive now exhibit tensile strength (TS) as high as 780 MPa or higher, preferably 980 MPa or higher, and more preferably 1180 MPa or higher. It has been a typical practice to form high strength steel sheets into shapes that need light working; however, a possibility of forming them into parts with complicated shapes is now being investigated.
Typically, steel sheets tend to show lower formability as their strength is increased. A challenge associated with use of high strength steel sheets is how to suppress cracking in press-forming. In particular, bendability is important for high strength steel sheets with a tensile strength of 780 MPa or higher since there are more parts to be worked by bending. Due to gauge down of parts associated with increasing strength, there may be more parts that require plane bending-fatigue properties higher than conventionally required. Moreover, a high strength steel sheet with a tensile strength of 780 MPa or higher achieves the high strength by increasing the amounts of C, Mn, etc., to be added in order to obtain a particular amount of martensite and by increasing the amount of Si added to solution-strengthen the ferrite phase. Since Si is oxidizable element prone to oxidization compared to Mn and Fe, ensuring zinc coatability and surface appearance quality is an issue in producing a galvanized steel sheet or galvannealed steel sheet that contains large quantities of Si and Mn. That is, Si and Mn contained in the steel undergo selective oxidation even in a non-oxidizing atmosphere or a reducing atmosphere used in a typical annealing furnace and thereby form oxides by surface segregation. The presence of these oxides of Si and Mn degrades wettability of molten zinc to the steel sheet in the coating process and bare spots may occur as a result.
To address this issue, in Patent Literature 1, a steel sheet is preliminarily heated in an oxidizing atmosphere so as to rapidly generate an Fe oxide film on a surface at a particular oxidation rate or higher to thereby prevent oxidation of additive elements on the steel sheet surface, and then the Fe oxide film is reduced by reduction annealing to improve wettability to molten zinc. However, if the amounts of oxides in the steel sheet is high, iron oxide may adhere to rolls in the furnace and pressing flaw may appear on the steel sheet, which is a problem.
Patent Literature 2 discloses a galvanizing method that involves pickling a steel sheet after annealing to remove oxides on the surface and then again annealing the pickled steel sheet. However, although Patent Literature 2 describes a steel sheet having strength of the 590 MPa TS grade, it makes no mention of steel sheets having TS of 780 MPa or higher, bendability, or fatigue properties.
Patent Literature 3 discloses a method for producing a high strength galvanized steel sheet having excellent bendability and fatigue properties and a tensile strength of 980 MPa or higher, in which the ferrite phase accounts for more than 70% of a microstructure of a steel sheet surface portion that extends from the surface of the steel sheet to a depth of 10 μm, the ferrite fraction of a steel sheet inner layer portion that extends from the depth of 10 μm from the surface toward the interior is 20% to 70%, and the average crystal grain diameter is 5 μm or less. According to this technology, the air ratio during the primary heating process from 200° C. to an intermediate temperature of 500° C. to 800° C. is adjusted to 1.10 to 1.20 so that Fe oxides are formed on the steel sheet surface, oxygen in the oxides bonds to C in the steel to decrease the amount of dissolved C, and the ferrite volume fraction increases in the steel sheet surface layer portion only. The literature describes that bendability is improved without degrading fatigue properties in this manner. However, when a high air ratio is used during the annealing process, the oxides that segregated on the surface decrease the wettability to the molten zinc during the coating process and bare spot may be created. Moreover, the iron oxides may adhere to the rolls in the surface and pressing flaw may appear on the steel sheet. Furthermore, in Patent Literature 3, bendability is evaluated by visually observing presence/absence of cracks on the outer side of the bend, obtaining a minimum bend radius that does not generate cracks, and determining the ratio of the minimum bend radius to the thickness. However, as the strength of the steel sheet increases, evaluation of bendability has become more stringent; for example, presence/absence of cracks must be evaluated by using a magnifying glass or a microscope. Small cracks which have not been considered as cracks in the conventional art must also be evaluated.
Patent Literature 4 discloses a high strength galvanized steel sheet that has a tensile strength of 980 MPa or higher and excellent formability, weldability, and fatigue properties. This high strength galvanized steel sheet contains less C, P, and S from the viewpoints of formability and weldability and less Cr and more Si from the viewpoint of fatigue properties, and contains a ferrite phase having a volume fraction of 20% to 70% and an average crystal grain size of 5 μm or less. While presence/absence of cracks on the outer side of the bend is visually observed to evaluate bendability in Patent Literature 4, evaluation of bendability has become more stringent as the strength of the steel sheet increases. For example, presence/absence of cracks must be evaluated by using a magnifying glass or a microscope. Small cracks which have not been considered as cracks in the conventional art must also be evaluated. Furthermore, in order to improve fatigue properties, 0.35% or more and less than 0.80% of Si is added; although Patent Literature 4 describes bare spots can still be avoided at this amount, Si is more prone to oxidation than Fe and thus ensuring coatability arises as an issue in actual procedures. However, Patent Literature 4 does not mention any specific techniques for ensuring coatability and description related to evaluation of coating surface appearance quality is not found in Examples also.
Patent Literature 5 and Patent Literature 6 each disclose a galvanized steel sheet having a tensile strength of 780 MPa or higher and excellent bendability and fatigue strength. This galvanized steel sheet has a polygonal ferrite microstructure and a low-temperature transformed microstructure; and when a plane at a depth 0.1 mm from the steel sheet surface is observed by changing positions in the sheet thickness direction with a microscope to obtain 20 view areas, the maximum value of the polygonal ferrite area fraction is 80% or less and the minimum value is 10% or more, and the difference between the maximum value and the minimum value is 40% or less. However, in evaluating the bendability, one side of a test piece is fixed with a die and a punch is brought down at a clearance equal to a test piece thickness +0.1 mm so that 90° bending can be performed along the radius of the die shoulder. According to this procedure, the apex of the bend is subject to a different deformation mode than that in the 90° bend test according to a V-block bend method prescribed in JIS Z 2248 and therefore the minimum bend radius may sometimes be evaluated smaller (in other words, evaluated to have high bendability) than in the V-block bend method. Moreover, presence/absence of cracks is checked by observation with a magnifying glass and judged on the basis of whether hair cracks occur. However, the magnifying power of the magnifying glass is not disclosed and the hair cracks are not specifically defined. For example, assuming that the hair cracks are cracks with size of a diameter of a strand of hair, that size is about 0.1 mm which will appear to be 3 mm if observed with a 30× magnifying glass. If a 30× magnifying glass is employed, it is possible that smaller cracks can be observed. Considering the current trends of increasing severity of the bend evaluation method associated with the increase in strength of the steel sheet, evaluation of smaller cracks is necessary. Furthermore, Patent Literature 6 does not include descriptions related to the technology for ensuring coatability. Patent Literature 5 only describes that the Si content is adjusted to less than 0.6% (preferably 0.5% or less and more preferably 0.3% or less) in order to use the steel sheet for galvanizing without a special facility (Fe pre-coating or the like). No examples describe evaluation of the surface appearance quality.